Brain games can help with abstract reasoning months after the training, but they work only for those who really need and enjoy the exercises, a study says.

Training a child to hold a whole cluster of items in his or her memory for even a short time may feel like trying to hold a wave on the sand. But a study published Monday says it's a drill that can yield lasting benefits.

Children who've had such training have better abstract reasoning and solve problems more creatively than kids who haven't, the study found.

But here's a warning to parents already grooming their young children for entry into elite universities: Don't automatically rush out to enroll your young genius in brain-training summer camp or invest in DVDs promising to deliver high IQs. These drills, the scientists found, pay the greatest dividends for children who actually need them and who find the escalating challenge of the games fun, not frustrating.

For others, "it might be difficult if you push your kid too much," said study lead author Susanne M. Jaeggi, a psychology professor at the University of Michigan. "It's like a parent pushing a child to do sports or learn a musical instrument: There's always this delicate balance between too much or too little."

The training program used by Jaeggi and co-workers focused on ramping up working memory: the ability to hold in mind a handful of information bits briefly, and to update them as needed. Cognitive scientists consider working memory a key component of intelligence. But they have long debated whether strengthening short-term memory capacity will boost a person's overall intellectual function, and will do so even after the brain-training sessions are over.

It can, and it does, according to this new research, published in the Proceedings of the National Academy of Sciences.

The study put 32 elementary and middle school children through a rigorous monthlong regimen of computer games designed to test, challenge and strengthen their working memory. An additional 30 children trained on a computer program that involved answering general knowledge and vocabulary questions.

The working-memory programs — adapted from a brain game designed for older users — required children to follow and remember a sequence of positions on a grid and, shortly after seeing the pattern, to answer questions about it. When a child did well on a game, the next sequence would become longer, increasingly challenging the child's ability to hold in mind the sequence and spatial information.

The task requires a child's rapt attention for as long as a minute and emphasizes the ability to screen out distractions while focusing on a single task. The child must recall where and in what order items appeared on a screen, then work backward through that remembered information to answer questions correctly.

Jaeggi called the task, known as the "n-back test" by psychologists, "really devilish. If you lose track just a little bit, you're completely out of it and you have to start anew."

When the children were tested at the end of the month of training, the Michigan researchers at first found scant differences between the group that got the working-memory training and the general knowledge group. Although those who had received working-memory training were better at holding several items in mind for a short while, on a test of abstract reasoning — fluid intelligence — they were, as a group, no smarter than the control group.

But then the researchers took a closer look and noticed a clear pattern: The children who had improved the most on the memory-training task did indeed perform better on the fluid intelligence test. And three months later, they still did better as a group than both the control group and the children who hadn't improved.

The study comes against the backdrop of explosive growth in the business of brain-training programs for children. Increasingly, designers of brain games — a roughly $300-million-a-year business that has sprung up in less than a decade — are aiming at intellectually ambitious parents bent on supplying their progeny all the cognitive advantages money can buy.

Alvaro Fernandez, who teaches the science of brain health at San Francisco State University and is the founder of SharpBrains, a company that tracks the brain fitness business, said about $75 million a year of the brain-training business was focused on school-age children. In a deal certain to accelerate that trend, the educational publishing giant Pearson last year bought Cogmed, a Swedish start-up company that has pioneered the development of brain-training programs focused on working memory.

"They'll have a sales force in every school district in the country," Fernandez predicted.